Marine Solar Planning Guide

Typical marine solar panels are comprised of a number of silicon cells (normally 32+) connected together electrically in a series string. Individual silicon cells produce only around 0.6v to 0.7v, and so enough of them have to be connected together in series to produce a voltage high enough to be able to charge a 12v battery.

A Charge Controller must be connected between the panel and the battery to reduce the panel output to a safe charging voltage. Some panels have less than the normal number of cells and produce less voltage than is required to charge a 12v battery, and these will require either a special boost controller, or for a number of them to be connected in series to produce a higher voltage.

Panel voltage is dependent on the number of cells, and the temperature of the cells. A typical 36 cell panel will produce about 20v at room temperature, but less as the cells get hotter in bright sunlight.

Panel current is dependent on the size, type, and quality of the cells, and the strength and quality of the available light. A typical 5” (125mm) squared monocrystalline cell produces about 5.5 amps in good sunlight.

All types of solar panels with silicon cells will give similar performance in full sun at solar noon, but monocrystalline cells generally perform better in shade and low light situations than polycrystalline.

In marine installations we are more interested in the potential accumulated output over the course of a complete day, than in an instantaneous power figure. Unfortunately, there are no ratings available for daily yield, so choosing the best panels for a boat installation requires some research.

For the maximum output over the course of a day, choose panels and controllers that will give the best shade and low-light performance. Panels utilizing genuine, high-grade SunPower® cells have the highest efficiency as well as higher heat tolerance and exceptional low-light performance.

Sizing Solar Panels:

Solar panels on boats can either provide a small trickle charge to prevent the battery from discharging while the vessel is unattended, or they can provide all or a portion of the total daily power requirements.

A single, small panel is all that is typically required to keep a battery topped-up and take care of an occasional bilge pump operation, but solar installations intended to provide power for everyday needs will require a larger panel, or more likely, an array of several large panels.

An estimate of the daily amp/hr demand must be calculated before being able to assess what size of array is suitable or practical, and battery size is not important at this stage.

You will first need to know how much power your vessel is using before being able to estimate how much you can hope to replenish with solar power.

Temperature – The hotter the cells get, the lower the voltage, and hence the lower the panel output. Panel voltage can easily drop 3v from the rated voltage (which is given at room temperature, 25C/77F), once the cells heat up in bright sunlight. Current output actually goes up very slightly at higher cell temperatures.

Quality and Quantity of sunlight

Soft Shading – Shade from rigging, etc., will reduce the current output of a silicon cell. As the cells are connected electrically in a series string, shading that affects any portion of any one cell will affect the whole panel proportionally.

Some panel manufacturers use smaller cells in an effort to make their panels “shade tolerant”, but the smaller the cell size, the smaller the shadow required for it to be 100% shaded. Shadows from the mast, boom, rigging, etc. on sailboats typically extend across the whole panel, and so panels made up of several “sub panels” connected in parallel, each fitted with small cells, handle shading poorly compared to panels with a single string of full-size cells.

Hard Shading – If a cell is completely covered by something opaque, i.e. a portion of canvas, a towel, plastic bag, etc, there is a danger of cell damage from overheating. Silicon solar cells consume energy as well as produce it, and if a cell is covered, the other cells in the panel will feed power in to it. If this power is great enough, it can cause a “hot spot” and literally burn through the cell and the substrate. To prevent this, most panel manufacturers install By-Pass Diodes that will shunt dangerous currents around a section of the panel where a cell is hard shaded. It is generally considered that any panel or section of panel less than 50w will not cause a cell to burn if hard-shaded, and so By-Pass Diodes, or “Hot Spot Preventers” are only installed across sections of panels of 50w and above. If the light to a cell is completely blocked and a By-Pass Diode “switches on”, the panel voltage will be reduced in proportion to the number of cells that have been by-passed, but this prevents the cell, and possibly the vessel, from burning. By-Pass Diodes do not consume energy, and are an important safety feature that should be installed all marine panels over 50 watts.

Location – In Summer, the lower the latitude, the stronger the irradiance but the shorter the solar day and the hotter the cell temperature. Higher latitudes experience lower levels of irradiance but have longer solar days and cooler cell temperatures. In fact, in Summer months, the same amount of daily irradiance is available all along the entire East coast of the USA, from southern Florida to northern Maine.

Panel Angle – To realize the maximum power output, solar panels should ideally be positioned perpendicular to the sun at all times. While this is possible to engineer into large land arrays, it is mostly impractical to implement successfully on a small boat that is moving around. In Southern Florida, the loss from having a horizontally mounted panel versus one at the ideal angle to the sun is only about 7%, and this loss diminishes the further south toward the equator one travels.

Multiple Panel Configuration – If there is a possibility that any one panel in an array of multiple panels could be shaded, the only way to ensure maximum power output is to use one controller per panel. A configuration of several panels connected in a series string to one controller should only be considered where there is absolutely no chance of shadows, as any shading of just one cell will reduce the total output of all the panels combined. A single controller used for multiple panels connected in a parallel configuration will be working on a compromised mix of all the panels’ outputs when shading occurs. Also, panels connected in a parallel configuration must have blocking diodes installed to prevent power back-feeding from an unshaded panel into a shaded one. Blocking diodes reduce panel voltage by 0.7v.

Panel size – The trend with land-based solar arrays is to use increasingly larger panels with a large number of cells resulting in a high voltage output. Where this is a very practical solution on land in a location where shade will not be an issue, any form of shade on any one cell anywhere on the panel will cause a proportional decrease in total output. Multiple, smaller panels are recommended for marine applications where shade is expected and/or unavoidable.

Wiring size – Wiring from panel to controller must be sized for minimum volt-drop but within practical limitations. Most solar controllers are required to be mounted in a location which is at the same temperature as the batteries, as they alter the charging parameters according to the ambient temperature. Locating the controller near the battery also minimizes any volt drop between them, and so ensures good charging regulation. Most solar cable is AWG 10 and is good for a 50’ run with 3% volt-drop at nominal 12v.

Controller type - Solar controllers ensure that a battery is not subjected to excessive voltages, and also prevent back-feeding at night. Pulse Width Modulation (PWM) models allow full power from the panel to the battery until the battery voltage reaches a pre-determined upper limit, and then pulse the battery with the panel output at varying rates to prevent the battery voltage from rising any higher.

The more efficient and effective Maximum Power Point Tracking (MPPT) models employ sophisticated circuitry and algorithms that enable them to track the best mix of voltage and current to yield the maximum power possible. Under reduced sunlight and soft-shading conditions, a good MPPT controller can yield at least 30% more charging power than a PWM controller, and these controllers actually put more amps into the battery than a panel(s) produces!

Rating Solar Panels

Solar panels are rated in Watts under specific test conditions known as Standard Test Conditions (STC). These are: 1,000 watts per square meter (W/m2) of irradiance (solar energy); a cell temperature of 25C (77F); and an air quality of AM 1.5.

This combination of ideal conditions will only occur rarely and momentarily, but the resulting power output data is published so that equipment, cabling, fusing etc. can be sized to handle it safely.